Function walk_ifd 

pub fn walk_ifd<'a>(
    data: &'a [u8],
    base: u64,
    limit: u64,
    lay: Layout,
    at: u64,
    kind: Kind,
    name: String,
    depth: u8,
    rules: &dyn Rules,
) -> Walk<'a>

Walk the directories starting at the absolute offset at, which has no TIFF header of its own (a maker-note IFD): offsets inside are relative to base, and lay gives the byte order. Every position stays below limit; the first directory is at pointer depth depth.